Typically, a solid oxide fuel cell (SOFC) having a seal free (sealless) structure employs an electrolyte made up of an ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly. The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, a predetermined number of electrolyte electrode assemblies and separators are stacked together to form a fuel cell stack.
The operating temperature of the fuel cell is high, about 800° C. Therefore, when the reacted fuel gas containing unconsumed reactant gases therein (hereinafter also referred to as the off gas) is discharged to an area around the fuel cell stack, and is mixed with an oxygen-containing gas to induce combustion, the temperature of the fuel cell stack becomes high locally. Under these circumstances, durability of the fuel cell is lowered, and operation of the fuel cell stack cannot be performed stably. Further, since the temperature of the exhaust gas after combustion becomes higher than the operating temperature, the temperature difference between the oxygen-containing gas supplied to the fuel cell stack prior to the power generation reaction and the exhaust gas becomes excessively large. Therefore, a significant non-uniform temperature distribution occurs within the fuel cell stack, and power generation performance is degraded undesirably.
In this regard, a solid oxide fuel cell, as disclosed in Japanese Laid Open Patent Publication No. 2005-85520, is known. As shown in FIG. 23, the fuel cell is formed by stacking a power generation cell 1, a fuel electrode current collector 2, an air electrode current collector 3, and separators 4a, 4b. The power generation cell 1 includes a fuel electrode layer 1b, an air electrode layer 1c, and a solid electrolyte layer 1a interposed between the fuel electrode layer 1b and the air electrode layer 1c. The fuel electrode current collector 2 is provided outside the fuel electrode layer 1b, and the air electrode current collector 3 is provided outside the air electrode layer 1c. 
The separator 4a includes a fuel gas channel 5 for supplying a fuel gas substantially from a center portion of the surface of the separator 4a, which faces the fuel electrode collector 2. The separator 4b has an oxygen-containing gas channel 6 for supplying an oxygen-containing gas from the separator 4b, which faces the air electrode current collector 3.
Although not shown, a ring shaped metal cover covers the outer circumferential portion of a circular porous metal body, wherein a large number of gas discharge holes 7 are provided over the entire side portion of the cover.
In this structure, gas is discharged from the outer circumferential portion of the fuel electrode current collector 2 only through the gas discharge holes 7. Thus, the fuel gas diffuses into the porous metal body and is not emitted from the entire outer circumferential portion of the fuel electrode current collector 2. According to the disclosure, an amount of the fuel gas, which is not used during power generation, and which is discharged from the outer circumferential portion is suppressed, and thus it is possible to prevent back diffusion of air toward the fuel electrode.
However, in the above conventional technique, the separator 4a is stacked on the fuel electrode collector 2, whereby the exhaust gas discharged from the gas discharge holes 7 is combusted near the separator 4a, and thus the temperature of the exhaust gas becomes significantly high. Hence, the temperature difference between the region near the inlet where the oxygen-containing gas is supplied and the region near the outlet where the exhaust gas is discharged becomes large. Therefore, a significant non-uniform temperature distribution occurs in the separator 4a, and power generation performance is degraded undesirably.